AN3973

Power factor section

Doc ID 022164 Rev 1

7/24

2.1.1

Input capacitor

The input filter capacitor (Cin) must reduce the high frequency voltage ripple across Cin and

the switching noise due to the high frequency inductor current ripple. The input capacitor

depends on the voltage ripple (r) and it is usually between 5% and 20% of the minimum

input voltage:

●

Ripple voltage coefficient (%):

●

Input capacitor

In the applicative conditions, a polyester capacitor was chosen. It offers benefits in terms of

voltage stability and power factor (PF).

2.1.2

Output capacitor

The output capacitor is a function of voltage ripple (

●

Output capacitor

An electrolytic capacitor has been selected because it has low impedance (ESR) and

therefore provides good energy storage and improves the transient performance.

2.1.3

Boost inductor

The inductor boost depends on the several parameters and different approaches which can

be used. First, the inductor value is usually calculated so that the minimum switching

frequency is greater than the maximum frequency of the L6562A internal starter. Assuming

unity PF, it is possible to write:

●

Instantaneous switching frequency

Therefore, the inductor value is determined at the top of the sinusoid (

) where the

switching frequency is the minimum:

●

Inductor value

The optimum dynamic performances of the MOSFET device, selected for this application,

have allowed higher operation minimum frequency and consequently lower boost

inductance. In this case a 1.8 mH boost inductance was selected.

2.1.4

Power MOSFET selection

MOSFET SuperMESH3 technology perfectly matches boost converter characteristics. In

fact, this technology has got a dynamic performance better than that of the standard

SuperMESH but not as good as MDMESH. That translates into having low switching power

losses and commutation which is not so fast that it implies ringing phenomenon such as

decreased PF and THD factors. MOSFET selection is based mainly on maximum voltage

rating, total power losses, and maximum operating temperature. In this case a MOSFET

1

.

0

r

=

 

nF

100

r

*

V

*

f

2

I

C

min

AC

min

SW

in

IN

≈

π

=

 

uF

47

V

V

*

f

4

P

C

out

out

main

out

Out

≈

Δ

π

=

out

AC

Out

2

AC

IN

sw

V

))

sin(

V

2

V

(

V

LP

2

1

f

θ

−

=

 

2

π

=

θ

 

IN

out

min

SW

AC

Out

2

AC

AC

P

*

V

f

2

)

V

2

V

(

V

)

V

(

L

−

=